Nearly 30 hereditary disorders in humans result from an increase in the number of copies of simple repeats in genomic DNA. These DNA repeats seem to be predisposed to such expansion because they have unusual structural features, which disrupt the cellular replication, repair and recombination machineries. The presence of expanded DNA repeats alters gene expression in human cells, leading to disease.
One such hereditary disorder is Huntington's disease (HD). HD is a fatal, neurodegenerative disorder with no cure that is associated with cognitive decline, dementia, and loss of motor coordination. It is characterized by the progressive and heritable increase in length of CAG trinucleotide repeats that encode a polyglutamine tract, in the coding region of the Huntington (HTT) gene. These repeats can increase in number from one generation to another. The normal allele of the HTT gene contains less than 36 CAG repeats, whereas the mutant allele contains more than 36 repeats. Most HD patients carry one normal allele and a mutant disease-causing allele. Functionally, the aberrant accumulation of CAG repeats is thought to confer a toxic gain-of-function to the mutant HD protein, causing it to aggregate, form protein deposits (i.e., inclusion bodies), and induce cell death. Disease severity generally reflects the extent of expanded repeats in the mutant HTT protein.
Therapeutic options for HD include small molecule drugs like haloperidol, tetrabenazine, clonazepam, fluoxitine, and sertraline that are designed to control the phenotypic manifestations of the disease. While these drugs can improve quality of life for patients with HD, they are not expected to significantly reverse or alter disease progression or increase life expectancy nor do they address the underlying molecular mechanisms of the disease.
Another disease characterized by an expanded nucleotide repeat in genomic DNA is amyotrophic lateral sclerosis (ALS). ALS is a fatal neurodegenerative disease characterized clinically by progressive paralysis leading to death from respiratory failure, typically within two to three years of symptom onset. ALS is the third most common neurodegenerative disease in the Western world, and there are currently no effective therapies. A proportion of ALS patients are characterized by a large hexanucleotide (GGGGCC) repeat expansion, for example, in the C9ORF72 gene (see, e.g., Renton et al., Neuron 2011; 72:257-68 and DeJesus-Hernandez et al., Neuron 2011; 72:245-56).
Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are associated with long polyCUG and polyCCUG repeats in the 3′-UTR and intron 1 regions of the transcript dystrophia myotonica protein kinase (DMPK) and zinc finger protein 9 (ZNF9), respectively. While normal individuals have as many as 30 CTG repeats, DMI patients carry a larger number of repeats ranging from 50 to thousands. The severity of the disease and the age of onset correlates with the number of repeats. Patients with adult onsets show milder symptoms and have less than 100 repeats, juvenile onset DM1 patients carry as many as 500 repeats and congenital cases usually have around a thousand CTG repeats. The expanded transcripts containing CUG repeats form a secondary structure, accumulate in the nucleus in the form of nuclear foci and sequester RNA-binding proteins (RNA-BP).
There is a need for new and/or improved therapeutic approaches to treating expanded repeat diseases. One possible route to treating expanded repeat diseases, such as HD and ALS, would be selective reduction or elimination of the gene product of a mutant disease-causing allele which contains expanded repeats.